A polyamide has excellent mechanical properties and processability and has relatively strong gas-barrier performance, and is therefore widely used not only for injection-molding materials for automobile parts, electric-electronic parts and others but also for wrapping/packaging materials for foods, drinks, chemicals, electronic parts and others as well as for industrial materials. For example, a polyamide to be obtained through polycondensation of a xylylenediamine and an aliphatic dicarboxylic acid has high strength and elastic modulus and has low permeability for gaseous substances such as oxygen, carbon dioxide gas, odor, flavor and the like, and is therefore widely used as gas-barrier materials in the field of wrapping/packing materials. In addition, a polyamide has resistance and has gas-barrier performance against alcohols, weakly-alkaline chemicals, weakly-acidic chemicals, fuels, various types of organic solvents, industrial gasses and the like, and is therefore also widely used for materials in the field of industrial application.
Various methods for production of polyamide resins are known. For example, there is known a method that includes: obtaining a polyamide through polycondensation of a dicarboxylic acid component and a diamine component; and then increasing the degree of polymerization of the polyamide according to a solid-phase polymerization method using a heating and drying apparatus such as a tumble drier or the like. However, the solid-phase polymerization method is limitative in producing a resin having a high melting point or in increasing the molecular weight of a resin having low crystallinity, and is therefore problematic in that the method could not provide a resin having a high melting point.
On the other hand, also known is a method of obtaining a polyamide resin by melt-kneading a polyamide oligomer that has been obtained through polycondensation of a dicarboxylic acid component and a diamine component using a twin screw extruder for further polycondensation of the resultant oligomer (for example, see Japanese Patent Document JP-A 2012-188557). The advantages of the twin screw extruder are that an oligomer can be melt-kneaded therein within a short period of time and, in addition, a polyamide resin having a high melting point can be produced, and further, many kinds of products in small quantities can be processed owing to the self-cleaning performance thereof. Accordingly, various methods for producing various types of polyamide resins have been investigated using a twin screw extruder.